The present invention relates to hydrostatic transaxles intended primarily for use in the lawn and garden industry on riding lawnmowers, lawn and garden tractors and the like, but may also be applied to larger implements and vehicles.
Hydrostatic transmissions transmit rotary mechanical motion, typically from an internal combustion engine, to fluid motion, typically oil, and then back to rotary mechanical motion to rotate a pair of drive axles in order to drive the vehicle. The hydrostatic transmission controls the output rotary mechanical motion such that varying output speeds in the forward and reverse directions are possible with a single speed input rotary mechanical motion. Such transmissions have utilized radial piston pumps and motors, axial piston pumps and motors and hybrid transmissions wherein the pump may be of the radial piston design, for example, and motor formed as a gear pump. The speed of the output of the transmission is typically controlled by varying the eccentricity of the pump track ring or swash plate.
In recent years, and particularly for smaller displacement applications, it has been common practice to integrate the hydrostatic transmission within the axle casing that also contains the differential and bearings for the two axles. Such casings are typically split along a horizontal plane containing the axis of the axles, and the casing itself is formed of only two parts. However, it is also known to utilize casings comprising three or more components wherein the casing components are attached to each other along vertical split lines or horizontal and vertical split lines.
Although utilizing a single casing for both the transmission and axle gear components necessitates only two large die castings, there are certain disadvantages inherent in such a design. One such disadvantage is that servicing of the hydrostatic transmission or the gear train/differential requires that the entire casing be opened, the oil drained and the complete mechanism withdrawn in order to perform such service.
Furthermore, units wherein the hydrostatic transmission is sized for different displacements, for example, use with larger lawn and garden tractors, will require completely different transaxle casings. Since such casings are quite large and require expensive tooling to manufacture, this represents a significant additional cost.
Furthermore, integrated hydrostatic transaxles are often noisy, especially when accelerating.
A typical construction of the hydrostatic transmission component of the transaxle includes a one-piece block common to both the pump and motor units, often referred to as a xe2x80x9ccenter section.xe2x80x9d The center section facilitates an external mounting surface for a motor barrel and a pump barrel, and additionally, an internal valve body for providing hydraulic communication between the pump and motor barrels. Conventionally, the pump and motor barrel axes of rotation are 90 degrees to one another. Center section machining is difficult because the center section is substantial in size and machined surfaces are substantially perpendicular, often requiring multiaxis machining capabilities. This corresponds to a significant cost associated with this design type.
Moreover, many HSTs heretofore require that the pump and motor mechanism unit be matched to a fixed swash plate prior to mounting the mechanism into the casing. Typically, assembly requires positioning the fixed swash plate in the casing, mounting the pump and motor mechanism into the casing then taking measures to ensure the fixed swash plate is suitably aligned with the pump and motor mechanism. Arranging the fixed swash plate and pump and motor mechanism, in the manner described above, poses a significant step in the assembly process which represents additional cost.
The present invention is a modular transaxle providing an axle module including an axle casing, a differential and a reduction gear train connected to a differential. The differential and gear train are disposed in the axle casing and a pair of axles are connected to the differential and supported within the axle casing. A hydrostatic transmission module comprising a transmission casing separate from the axle casing hydraulically connects a pump and motor disposed in the transmission casing. An input drives the pump and the motor has an output connection to drivingly connected the reduction gear train to the motor. The hydrostatic transmission casing is removably connected to the axle mechanism casing. The motor output connection includes a shaft piloted to an input drive of the reduction gear train. The piloting of the shaft and input drive constitutes the alignment mechanism of the transmission and axle modules.
The present invention further provides a hydrostatic transmission including an axle module having a casing, a differential and a reduction gear train connected to the differential. The differential and gear train are provided in the axle casing and a pair of axles are connected to the differential and are supported within the casing. A hydrostatic transmission module comprises a casing separate from the axle module casing and includes a hydraulically connected pump and motor within the transmission casing. The pump has an input for driving the pump and the motor has an output connection attaching the reduction gear train to the motor. The hydrostatic transmission casing is connected to the axle casing and the motor output connection includes a shaft attaching to an input drive of the reduction gear train. The hydrostatic transmission module includes a pump and motor block rotatably supporting a pump cylinder barrel and a motor cylinder barrel. The pump and motor cylinder barrels are hydraulically connected through the pump and motor block and include axes of rotations substantially 90xc2x0 offset. A portion of the pump and motor block forms an inclined surface which supports one face of a thrust bearing. The other face of the thrust bearing is engaged with a plurality of reciprocal pistons in the motor cylinder barrel.
In one form of the invention, the axle casing has two major casing halves joined together at an interface, the casing halves are substantially mirror images relative to one another about the interface.
A swash plate is positioned between the pistons and an interior portion of the transmission casing and preferably at least one homogeneous low friction bearing strip is confined between the swash plate and the interior portion of the casing.
A disconnect mechanism removably connects the axle module with the hydrostatic transmission and is disposed within the differential casing. The disconnect mechanism include a sleeve, a lever and a resilient member. The output shaft is selectively engaged with the input shaft through the sleeve and the resilient member urges the sleeve to engage the output and input shafts.
A brake shaft having an end disposed within the axle casing is meshingly coupled to the reduction gearing and preferably includes a pair of friction pads sandwiching a brake disc. The brake disc is disposed on the other end of the brake shaft and provided externally of the axle casing. A brake cover is fastened to the outer portion of the axle casing and substantially encloses the brake disc.
The present invention further provides a transaxle system and a method for manufacturing transaxles in either the left hand controlled or right hand controlled configurations using substantially identical axle modules. The transmission module used in the left hand controlled transaxle has the transmission module having the output shaft on the left and the control lever on the right when viewing the transmission casing with the pulley upwardly directed and toward the rear of the casing. On the other hand, the transmission module used in the right hand controlled transaxle has the transmission module having the output shaft on the right and the control lever on the left when viewing the transmission casing with the pulley upwardly directed and toward the rear of the casing. The use of either the left hand controlled or right hand controlled hydrostatic transmission modules allows either the left hand controlled hydrostatic transaxle or right hand hydrostatic transaxle configurations to be constructed using identically manufactured axle modules that are inverted for left or right hand drives.
The modular arrangement enables the hydrostatic transmission to be quickly removed and a replacement unit bolted in place if the hydrostatic transmission requires repair. The transmission can be sent back to the factory for rebuilding, and only minimal downtime to the consumer""s lawn and garden vehicle will be experienced.
Another advantage to the modular approach is that a variety of hydrostatic transmissions and a variety of axle mechanisms can be mixed and matched to meet customers"" application requirements.
A further advantage is that the use of a separate casing for enclosing the hydrostatic transmission enables the hydraulic components to be much more tightly held in place, thereby reducing noise, which is a problem with many integrated transaxles wherein the single casing encloses both the transmission and reduction gearing/differential mechanisms.
In the particular embodiment disclosed herein, the hydrostatic transmission employs an axial piston pump having a vertical input shaft, and an axial piston motor having a horizontal output shaft that extends out of the transmission casing and engages the reduction gear train of the axle mechanism through a mechanical disconnect device.
A further improvement in the transaxle of the present invention is the use of a brake cover, which may be made of molded plastic, in order to protect the brake disc from debris.